Spot colors are recognized as standardized colors. Some of the known spot color classification systems include: Pantone®, Toyo, DIC, American Newspaper Publishers Association (ANPA), and HKS. Accurate printing of spot colors is important to achieve a good color match to the original hardcopy proof on paper.
Standard spot color proofs are typically supplied by vendors, such as Pantone®, in the form of samples which can be separated for measurement and viewing. They may also be supplied in the form of swatch books. Spot colors are often used, or can be used, for large background areas, which may be the most color critical portion of a particular page.
Many software packages have the ability to enter or create device dependant recipes, such as Cyan, Magenta, Yellow and Black (CMYK) recipes, using multi-dimensional profiles for a particular spot color and store them for future use. These software packages may also allow users to manually edit the recipe.
A problem, however, with this approach is that the CYMK recipe that was originally entered and/or obtained through manual entry may not be very accurate at the time of reproducing the spot colors due to (a) variations in the print engine state, (b) operator error associated with manual adjustments or modifications to CMYK recipe values; and (c) inaccurate recipes to begin with.
An automated software application “Spot Color Control” (SCC) based on the Xerox Corp.'s “Automated Spot Color Editing (ASCE)” technology is currently available for use with inline sensors for the Xerox Corp.'s iGen3® and DC8000® Digital Production Presses. This technology uses an iterative process to find the device specific recipe under a D50 illuminant. While the printed spot colors may appear matched to the hardcopy proof under D50 light source, those colors may not “look” the same when viewed under other light sources.
The conventional techniques used for matching spot colors are optimized to a 2 degree observer and a D50 light source. Thus, even if the spot colors are matched under the D50 illuminant, they may look different when viewed under different angles.
FIG. 1(a) shows color variations for a Pantone® 202C spot color under multiple illuminants. The spot color has a near perfect match under the D50 light source (i.e., delta-E values are approximately zero). However, the figure shows the expected delta-E variations across other D series illuminants (D series with correlated color temperature from 3600 to 7500, or D36 to D75), Stirling fluorescence lamp, white LED, and Firestar fluorescence lamp. The maximum spread across the selected group of illuminants is about 4.25 delta-E 1976 (“deltaEab”) and 2.41 delta-E 2000 (“deltaE2K”).
FIG. 1(b) shows color variations for a Pantone® 251C spot color, which is outside the gamut of a printer, across multiple illuminants when this out-of-gamut spot color is mapped for a gamut with D50 illuminant and a 2 degree observer.
FIG. 1(c) shows color variations for a Pantone® 3385C spot color which is again outside the gamut of a printer, across multiple illuminants when this out-of-gamut spot color is mapped for a gamut with D50 illuminant and a 2 degree observer.
A seasoned color expert (human) may be able to recommend which light sources are best to use for a specific color matching application. However, due to variety of light sources available this may be difficult. Also, the light sources used for viewing may vary depending on environmental factors (e.g., humidity, temperature, ambient lighting conditions, location and geometry of the light source, etc.), which are difficult to control.